Add a spec for autoload with private_constant

[Earlopain]

This recently regressed in JRuby. The autload should be accessible and only fire on actual access.

Ref jruby/jruby#8923

cc @headius

[headius]

This is a good start! I would like to get a review from @eregon as to whether some of this should be moved into fixtures rather than defined in line. I'm not a fan of having a tiny amount of code in the spec and a large amount of code in a fixture, but these are a little cumbersome.

It also occurs to me, the JRuby bug was caused by the private_constant method actually removing the autoload, or at least damaging it in such a way that it no longer functions. We could also test that the constant in question still shows up as being an autoload, without actually triggering it.

[eregon]

Looks good to me, I'll merge.

whether some of this should be moved into fixtures rather than defined in line.

The other autoload examples do things the same way so it's fine and in general good to match the existing style.